Seed Dispersal
Thinking like a plant….
•Coast redwood tree
•May live 2,000 years. May make 1-10 billion seeds in lifetime
•Sedentary, mindless
•Often long lifespan, Often high fecundity
•“Sweepstakes reproduction”
•Takes just one seed to replace itself!
Seed dispersal
•How seeds move from parent plant to “safe sites:” places suitable for germination and
seedling establishment
•Importance: may determine if species can track “climate envelopes” as they change due
to global climate change
Why disperse?
•1) Escape competition from parent/siblings
•2) Escape from predation/disease (if density-dependent)
•3) Discover new suitable habitats
•4) Contribute to gene flow/genetic diversity
•5) Maintain populations in “sink” habitats (metapopulations: source vs sink populations)
Seed plants: Seed is fertilized ovule in ovary of pistil
•Seed: baby plant (embryo) in box (seed coat) with lunch (endosperm in flowering
plants)
•Fruit: Mature ovary of flower (contains one or more seeds)
•Ovary wall becomes pericarp in fruit.
•Pericarp may develop specialized layers. At most these are:
–ectocarp: outer layer
–mesocarp: middle layer
–endocarp: inner layer
Fruits vs seeds
•Dry indehiscent fruits
–achene: one seed, fused to pericarp at one point
–Ex, dandelion, sunflower.
–grain (caryopsis): one seed, fused entirely to pericarp
–Ex, corn, rice, wheat.
Resolution
•Solution: diaspore. Single dispersal unit of plant (seed or fruit)
Fruit functions
•1) Fruit can be protective against:
–Seed predators
–Environmental conditions
•2) Fruit can promote seed dormancy
–Hard endocarp can seal out water/oxygen
–Breaking layer called scarification
–Fleshy fruits can contain germination inhibitors (prevent seed from germinating in fruit)
–Rotting or digestion by animal gut needed to remove inhibitors so seed will germinate
•3) Fruit can aid seed dispersal
•Wings, barbs, fleshy reward
Seed coat functions
•1) Protect embryo (like pericarp)
•2) Promote seed dormancy (like pericarp)
•3) Aid in seed dispersal
–Hairs on some wind-dispersed seeds
Ballistic dispersal
•Many types of seed dispersal are NOT mutualisms
–Ex, Ballistic dispersal=Ballistochory: fruit throws or squirts seeds
–Example, fruits of touch-me-not (Impatiens, right), sorrel (Oxalis, left)
•Other non-mutualist examples (physical forces)
•Anemochory=Wind dispersal (dandelion)
•Hydrochory=Water dispersal (coconut)
Ectozoochory
•Some types of biotic seed dispersal are NOT mutualisms
–Ex, most cases of ectozoochory. Seed or fruit carried by animal outside body (hooks,
barbs, glue)
–Ex, Devil’s claw (Proboscidea)
–Makes fruit with large hooked claws (elongated top of capsule).
–
–Many plant species use this technique
–Table shows the (unofficial) 17 top “hitchhiker plants,” ranked by SRDUs (Sock
Removal Difficulty Units)
•
•Ectozoochory: Some cases are mutualisms. Example, scatter hoarding animals
(squirrels, nutcrackers)
–Collect and hide seeds or fruits in caches
–Some escape and germinate
–Pinus albicaulis (white bark pine) and Clark’s nutcracker
–1 bird can hide as many as 90,000 seeds in one season
•
•Endozoochory: fruit eaten and seeds travel thru gut
–Common with fleshy fruits (soft, sweet fruits)
–Fruit pulp is nutritive reward to disperser
•
•Endozoochory: sometimes seed scarified
–Example, dodo
–Flightless “pigeon” found only on Mauritius in Indian Ocean
–Discovered by European visitors around 1600
•Had gizzard with stone against which food (seeds) ground
•Seeds of Calvaria tree have thick endosperm
•Apparently need scarifying to germinate, and dodo gizzard may have done this
•Problem: dodo extinct by 1680s!
•Tree seeds may not have germinated in nature since then
Myrmecochory
•Seed dispersal by ants (mutualism)
–Elaiosome: food body on outside of seed coat/fruit
–Ants collect seed, remove elaiosome, discard seed underground or aboveground
–Only major seed dispersal mode using an insect!
•Common for Eastern forest spring flowering plants.
Pollination vs biotic seed dispersal
•Trait
Pollen dispersal
•Animals
Often insects
• “Target”
Stigma
•Motivation Floral reward
•to target
•Cues to target
Floral traits
Seed dispersal
Mammals/birds
Safe site
None
•Seeds: “Directed dispersal” unlikely
Syndromes
•Protein rare reward (most plants N limited)
•Bird fruits with no odor
•Ants only major insect group
None
•Additional syndrome: mud dispersal (“sole botany”)
•43 species, no specific seed characteristics
•Includes 5 species of most common weeds:
•Polygonum (knotweed)
•Capsella (shepherd’s purse)
•Stellaria (chickweed)
•Chenopodium (lamb’s quarter)
•Poa annua (bluegrass)
•
•Less taxonomic specificity
•Guilds: groups of species with similar ecological function
•Ex, frugivore birds and summer-fruiting fleshy-fruited plants
The Plant View
•How measure dispersal? What’s the metric?
•1) Single species study: absolute distance
•2) Comparative study: canopy diameters
•Ex, grasses vs oaks
•3) Self-incompatible clonal plants: genetic neighborhood
Trade-offs
•Generally, as seed size increases dispersal decreases
•But chance of successful establishment increases
•
•Danger for Plant:
•Seeds are yummy! High protein, high lipid to supply embryo
•Solution 1: bribery!
•Use large numbers of seeds (scatter hoarders)
•Solution 2: bribery!
•Use non-seed food reward. Fruit pulp, etc
•Solution 3: poison!
•Defend seeds with toxins
•Ex, castor beans contain ricin
•Lethal dose: 1/5000 gram (twice as deadly as cobra venom!
•Solution 4: armor!
•Defend seeds with mechanical protection (stony seed coat, endocarp)
•Solution 5: advertise so correct animal gets message before seed predators arrive!
•Cues to fruit ripeness (color, smell, etc)
•Pre-ripening flags: signals ripening has begun (red color in ripening blueberries)
•Foliar flags: leaf color change indicates fruits ripening in fall (ex, poison ivy)